1. Field of the Invention
The present invention relates to an analog information storing and reproducing apparatus. More specifically, the present invention relates to an improved non-volatile field-effect memory capable of storing and reproducing analog information such as analog values or an analog signals.
2. Description of the Prior Art
Conventionally, non-volatile field-effect memories have been put into practical use for the purpose of storing and reproducing digital information in the form of logic values one and zero. To store and reproduce analog information, such as analog values or analog signals, with such non-volatile field-effect memories, it was necessary to convert such analog information into digital information with a plurality of bits by the use of an analog/digital converter, and then to store this plurality of bits of digital information in and reproduce it from a plurality of non-volatile field-effect memories. Thus, an analog/digital converter was indispensable in storing analog information in non-volatile memory devices of the field-effect type after analog/digital conversion and an increased number of such non-volatile field-effect memory devices was required in order to increase accuracy of stored and reproduced analog information. Furthermore, a digital/analog converter was also required to restore analog information from the digital information stored in such non-volatile field-effect memory devices.
The present inventors first thought of an approach to store an analog value directly in and reproduce it from one non-volatile field-effect memory without analog/digital and digital/analog conversion, by noting the fact that in a non-volatile field-effect memory device which stores information by electrically changing its threshold voltage a variable amount of the threshold voltage is changeable as a function of a write voltage.
To describe in more detail this approach thought of by the present inventors, it would be appropriate to first describe in more detail one example of such non-volatile field-effect memory devices.
FIG. 1 is a diagrammatic sectional view of a non-volatile field-effect memory device. The memory device shown in FIG. 1 is often referred to as a stacked gate avalanche MOS device and comprises a semiconductor substrate 10 of one conductivity type, such as a silicon substrate of an N-type, source and drain regions 11 and 12 of the opposite conductivity type, such as a P-type, formed on the substrate 10 and spaced apart from each other, thereby defining a channel region 14 therebetween, a silicon oxide film 15 of a thickness of about 200 .ANG. formed on the channel region 14, a floating gate 13 made of polycrystalline silicon highly doped with an impurity or metal such as molybdenum formed on the silicon oxide film 15 above the channel region 14, a silicon nitride film 16 of a thickness of about 1000 .ANG. formed to cover the floating gate 13, a control gate 17 formed on the silicon nitride film 16 above the floating gate 13, and source and drain electrodes 18 and 19 formed on and in ohmic contact with the source and the drain regions 11 and 12, respectively.
Such memory devices M have the floating gate 13 completely insulated by the silicon oxide film 15 and the silicon nitride film 16 and hence have a threshold voltage Vtm that varies as a function of the quantity of electric charge injected into the floating gate 13 by the use of some means, whereby information can be stored in the form of this charge. Such memory devices M were originally developed and have been used as a non-volatile memory in digital information applications, in view of the fact that storage of information in such a form is suited for digital information such as the logical values one and zero.
Meanwhile, the inventors of the present application thought of utilizing such memory devices M as analog information memories by noting the fact that the threshold voltage is variable as a function of the quantity of the electric charge injected into the floating gate 13 and hence the threshold voltage is dependent on the quantity of the injected electric charge. This thought by the inventors to use the above described memory device M as an analog information memory will now be considered in more detail.
FIG. 2 is a schematic diagram showing the use of the above described memory device M as an analog information memory and in particular, the use in a write mode thereof for directly storing an analog value in the memory device M without analog/digital conversion, which was first thought of by the present inventors. Referring to FIG. 2, a non-volatile field-effect memory device M having a floating gate FG is shown and, according to the original thought of the inventors, the control gate CG of the memory device M is connected to the ground, the source electrode S of the memory device M is kept open, and the drain electrode D of the memory device M is connected to receive a voltage of an analog value signal VS having a predetermined analog value at the time of writing superimposed on a critical voltage VW required for writing the information. As a result, a quantity of electric charge corresponding to the analog signal VS is injected into the floating gate FG of the memory device, whereby the analog value is stored in the form of the quantity of the electric charge.
However, it was observed that according to the method of writing analog information shown in FIG. 2, an electric charge with a magnitude proportional to the voltage (VW+VS) applied to the drain D is not necessarily stored in the floating gate FG. Because of diversification of the thickness and composition of the insulating films existing between the floating gate FG and the control gate CG, and between the floating gate FG and the substrate SB and, analog information cannot be stored with high accuracy. Hence, it is desired that analog information be directly stored in and reproduced from such non-volatile field-effect memory devices with high accuracy and without analog/digital and digital/analog conversion.
On the other hand, an analog signal such as an audio signal is most typically recorded on a magnetic tape and is reproduced therefrom by means of a magnetic tape recorder. Such tape recorders have been used for many years and have been much improved in many aspects. However, a fundamental structure of such magnetic tape recorders is a magnetic tape adapted to travel mechanically while an analog signal is electromagnetically recorded on the tape and then the analog signal recorded on the magnetic tape is read from the magnetic tape. Hence, such magnetic tape recorders inevitably involve the problem that a mechanical structure is required, resulting in lack of reliability and time dependent changes due to mechanical wear of the components. Another problem that is also involved in such magnetic tape recorders is that accessing a particular recorder portion in a magnetic tape requires much time, with the result that immediate access to any particular recorded portion of a magnetic tape is difficult. Such digital technologies referred to as PCM, PWM or the like have also been proposed as audio reproducing apparatus without inclusion of such mechanical structures. Furthermore, speech synthesis technology employing the PACOR approach, LSP and the like has also already been proposed and put into practical use. While employment of PCM, PWM or the like achieves the original intention of eliminating mechanical components, a huge amount of storage capacity is required for storing audio information and hence such an approach can be employed only in the case of large scale apparatus. On the other hand, a speech synthesis technology, such as the PACOR method, was proposed for the purpose of decreasing the storage capacity required for such PCM, PWM or the like, and was successful in terms of storage capacity. However, this method requires much work and time for extracting coefficients for speech synthesis from a speech to be reproduced and furthermore speech synthesis from such coefficients indispensably requires a processor such as a microcomputer, with the result that simplification of such apparatus is difficult and accordingly the use of the same for general purposes is disadvantageous. Nevertheless, as previously described, speech synthesis technology requires much time for extraction of the coefficients and is difficult to utilize by general users because of the incapability of recording and reproduction on a real time basis as compared to existing tape recorders. Accordingly, it is also desired that an improved analog information storing and reproducing apparatus is provided for accurately, simply, and directly storing and reproducing an analog signal that is variable as a function of time.